Clues about Mars' atmosphere may lie on the surface of its moon Phobos

An image of Mars' moon Phobos, taken by the High Resolution Imaging Science Experiment camera on NASA's Mars Reconnaissance Orbiter, on March 23, 2008.
An image of Mars' moon Phobos, taken by the High Resolution Imaging Science Experiment camera on NASA's Mars Reconnaissance Orbiter, on March 23, 2008. (Image credit: NASA/JPL-Caltech/University of Arizona)

The doomed Martian moon Phobos, which is on a slow death spiral above the Red Planet, may hold some secrets about how Mars was formed.

The moon's orbit of Mars — gradually degrading over the eons — brings it through charged molecules (ions) of oxygen, carbon, nitrogen and argon that Mars has been shedding from its atmosphere for billions of years. Some of those ions might remain on Phobos' surface, a new study shows.

The study was based on observations from NASA's Mars Atmosphere and Volatile Evolution (MAVEN) mission, which has been examining the Martian atmosphere since it arrived in the Red Planet's orbit in 2014. The spacecraft also crosses the orbit of Phobos five times every Earth day. 

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Related: Mars' moon Phobos looks like cosmic candy in these NASA photos

MAVEN's Suprathermal and Thermal Ion Composition (STATIC) instrument measured Martian ions in the orbit of Phobos, allowing scientists to then estimate how deeply these ions would plummet into Phobos' surface if they were to collide with the Martian moon. The researchers estimated that the ions dug no more than a few hundred nanometers deep, or 250 times shallower than the width of a human hair, according to a NASA statement.

"We knew that Mars lost its atmosphere to space, and now we know that some of it ended up on Phobos," lead author Quentin Nénon, a researcher at the Space Sciences Laboratory at the University of California, Berkeley, said in the statement. (A Berkeley team also designed and built the STATIC instrument.)

If somebody could study Phobos up close — no one has been able to get there yet — ion studies there could shed more light on the mystery of why Mars has lost so much atmosphere, and the associated question of when water stopped flowing on the planet's surface. Water is a key ingredient of life, and resolving the debate will help scientists better understand the Red Planet's chances for life. While MAVEN has been looking at atmospheric loss on Mars for years, Phobos would provide another perspective.

Phobos as observed by MAVEN's Imaging Ultraviolet Spectrograph. Orange shows mid-ultraviolet sunlight reflected from the surface of Phobos, exposing the moon's irregular shape and craters. Blue shows far-ultraviolet light, which is scattered off of hydrogen gas in Mars' upper atmosphere. (Image credit: CU/LASP/NASA)

Phobos has been observed from afar, but all efforts to reach the moon so far failed. Several planned missions never came to fruition, and even the launched ones didn't reach their destination (including the Soviet Union's Phobos 1 and Phobos 2 in the late 1980s, and the Russian-Chinese Phobus-Grunt mission that never managed to leave Earth orbit in 2011-12.)

But there is hope for more answers soon, as the Japan Aerospace Exploration Agency (JAXA) plans to send the Martian Moons Exploration (MMX) mission to Phobos in 2024. MMX will pick up samples from the surface and send them back to Earth. NASA said to answer Nénon's team's questions about the Martian atmosphere, it would be ideal if MMX landed on the side of the moon always facing Mars.

"Phobos is tidally locked to Mars, like Earth's moon is locked to Earth, thus always showing the planet only one side," NASA said in the same statement. "As a result, the rocks on the near side of Phobos have been bathed for millennia in Martian atoms and molecules. Nénon's research shows that the uppermost surface layer of Phobos' near side has been subjected to 20 to 100 times more wayward Martian ions than its far side."

Studying Phobos for clues about the Martian atmosphere has precedent, a lot closer to home. Earth's moon also has a record of atoms from both the sun and the Earth, which showed up in samples from the Apollo human landing missions of the 1960s and 1970s, NASA noted in the same statement. 

So-called "airless" moons such as Phobos, its twin Martian moon Deimos, and Earth's moon are subject to little erosion since there is no substantial atmosphere, or surface processes like wind and water. This slowly changing surface of these moons thus provides a precious record of solar system history.

Besides looking at the atmosphere of Mars, MMX may also help scientists understand the origin story of Phobos and Deimos. The tiny moons may be asteroids captured by Martian gravity, satellites of Mars formed at the same time as their parent planet, or leftovers from a cosmic collision.

A paper based on the new study was published Feb. 1 in Nature Geoscience.

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Elizabeth Howell
Staff Writer, Spaceflight

Elizabeth Howell (she/her), Ph.D., is a staff writer in the spaceflight channel since 2022 covering diversity, education and gaming as well. She was contributing writer for for 10 years before joining full-time. Elizabeth's reporting includes multiple exclusives with the White House and Office of the Vice-President of the United States, an exclusive conversation with aspiring space tourist (and NSYNC bassist) Lance Bass, speaking several times with the International Space Station, witnessing five human spaceflight launches on two continents, flying parabolic, working inside a spacesuit, and participating in a simulated Mars mission. Her latest book, "Why Am I Taller?", is co-written with astronaut Dave Williams. Elizabeth holds a Ph.D. and M.Sc. in Space Studies from the University of North Dakota, a Bachelor of Journalism from Canada's Carleton University and a Bachelor of History from Canada's Athabasca University. Elizabeth is also a post-secondary instructor in communications and science at several institutions since 2015; her experience includes developing and teaching an astronomy course at Canada's Algonquin College (with Indigenous content as well) to more than 1,000 students since 2020. Elizabeth first got interested in space after watching the movie Apollo 13 in 1996, and still wants to be an astronaut someday. Mastodon: